1. Field of the Invention
The present invention relates to a DC-DC conversion device which generates an alternating current voltage in a primary winding of a transformer based on an input direct current voltage from a direct current power source, and generates a direct current voltage by rectifying and smoothing an alternating current voltage generated in a secondary winding of the transformer.
2. Related Art
FIG. 3 is a circuit diagram showing a heretofore known configuration example of this kind of DC-DC conversion device. In the DC-DC conversion device, a series arm wherein semiconductor switch elements 101 and 102 are connected in series is connected in parallel to a direct current power source 1. Herein, a diode 111 and a capacitor 121 are connected in parallel to the semiconductor switch element 101, and a diode 112 and a capacitor 122 are connected in parallel to the semiconductor switch element 102. Further, a resonating reactor 3, a primary winding 21 of a transformer 2, and a resonating capacitor 4 are inserted in series between a common node between the semiconductor switch elements 101 and 102 and the negative electrode of the direct current power source 1.
As means for rectifying an alternating current voltage generated in a secondary winding 22 of the transformer 2, a full-wave rectifier circuit 13 of a full-bridge configuration formed of diodes 131 to 134 is connected on the secondary side of the transformer 2. An output voltage of the full-wave rectifier circuit 13 is smoothed by a smoothing capacitor 5 and output from the DC-DC conversion device.
An output voltage detection circuit 6 and pulse-width modulation control circuit 7 configure control means for controlling so that the voltage value of a direct current voltage output by the DC-DC conversion device maintains a target value.
More particularly, the output voltage detection circuit 6 is a circuit which detects the output voltage of the DC-DC conversion device. The pulse-width modulation control circuit 7 is a circuit which repeats the operation of generating a first pulse which turns on the semiconductor switch element 101, and subsequently, generating a second pulse which turns on the semiconductor switch element 102, in a predetermined cycle. The pulse-width modulation control circuit 7, having a pulse-width modulation function, carries out the control of an ON duty, which is the ratio of the pulse width of the first pulse in the cycles of the first and second pulses, in response to an increase and decrease in the output voltage, detected by the output voltage detection circuit 6, from the target value, and thus maintains the output voltage value of the DC-DC conversion device at the target value.
FIG. 4A is a waveform diagram showing an operation example of the DC-DC conversion device when at a low input voltage and under a heavy load, i.e., when the input direct current voltage given from the direct current power source 1 is low, and a load connected to the smoothing capacitor 5 is heavy, while FIG. 4B is a waveform diagram showing an operation example of the DC-DC conversion device when at a high input voltage and under a light or moderate load, i.e., when the input direct current voltage is high, and a load connected to the smoothing capacitor 5 is light or moderate. Each of FIGS. 4A and 4B shows the respective waveforms of a drain-source voltage V101 of the semiconductor switch element 101, a drain-source voltage V102 of the semiconductor switch element 102, a drain current I101 of the semiconductor switch element 101, a drain current I102 of the semiconductor switch element 102, a voltage V4 of the resonating capacitor 4, a voltage V21 of the primary winding 21 of the transformer 2, and currents I131, I132, I133, and I134 flowing respectively through the diodes 131, 132, 133, and 134. Hereafter, a description will be given, referring to FIGS. 4A and 4B, of an operation of the DC-DC conversion device shown in FIG. 3.
As heretofore described, the pulse-width modulation control circuit 7 alternately generates the first pulse which turns on the semiconductor switch element 101 and the second pulse which turns on the semiconductor switch element 102. When the semiconductor switch element 101 is turned on, a resonant current flows via a path from the direct current power source 1 through the semiconductor switch element 101, the resonating reactor 3, and the primary winding 21 of the transformer 2 to the resonating capacitor 4, and the resonating capacitor 4 is charged by the resonant current. During this time, a differential voltage between the input direct current voltage from the direct current power source 1 and the voltage V4 of the resonating capacitor 4 is applied to the primary winding 21 of the transformer 2 and the resonating reactor 3. Further, a voltage corresponding to the voltage V21 of the primary winding 21 is generated in the secondary winding 22 of the transformer 2, and the smoothing capacitor 5 is charged by the voltage via the diodes 131 and 134. Further, direct current power is supplied to an unshown load from the smoothing capacitor 5.
Next, when the semiconductor switch element 101 is turned off, the resonant current having flowed so far is commutated to the capacitors 121 and 122, and the drain-source voltages V101 and V102 of the semiconductor switch elements 101 and 102 rise or drop gradually.
When the drain-source voltage V101 of the turned-off semiconductor switch element 101 reaches the input direct current voltage from the direct current power source 1, the resonant current is commutated to the diode 112. At this time, by the semiconductor switch element 102 being turned on, a resonant current I102 flows via a path from the resonating capacitor 4 through the primary winding 21 of the transformer 2 and the resonating reactor 3 to the semiconductor switch element 102, and discharging of the resonating capacitor 4 is carried out by the resonant current I102. At this time, the voltage V4 of the resonating capacitor 4 is applied to the primary winding 21 of the transformer 2 and the resonating reactor 3. Further, a voltage corresponding to the voltage V21 of the primary winding 21 is generated in the secondary winding 22 of the transformer 2, and the smoothing capacitor 5 is charged by the voltage via the diodes 133 and 132. Further, direct current power is supplied to an unshown load from the smoothing capacitor 5.
Next, when the semiconductor switch element 102 is turned off, the resonant current having flowed so far is commutated to the capacitors 121 and 122, and the drain-source voltages V101 and V102 of the semiconductor switch elements 101 and 102 rise and drop gradually.
When the drain-source voltage V102 of the turned-off semiconductor switch element 102 reaches the input direct current voltage from the direct current power source 1, the resonant current is commutated to the diode 111. At this time, by the semiconductor switch element 101 being turned on, a resonant current flows via a path from the direct current power source 1 through the semiconductor switch element 101, the resonating reactor 3, and the primary winding 21 of the transformer 2 to the resonating capacitor 4, and the resonating capacitor 4 is charged by the resonant current.
By this kind of operation being repeated, another direct current power isolated from the direct current power source 1 is generated based on the input direct current power from the direct current power source 1, and supplied to an unshown load via the smoothing capacitor 5.
Herein, when at a low input voltage and under a heavy load, the semiconductor switch elements 101 and 102 each operate with an ON duty of on the order of 0.5, as shown in FIG. 4A, and the current I101 flowing through the semiconductor switch element 101 and the current I102 flowing though the semiconductor switch element 102 change into a sine wave shape.
When the load of the DC-DC conversion device changes, and the output voltage value of the DC-DC conversion device is off the target value, the pulse-width modulation control circuit 7 changes the respective pulse widths of the first pulse which turns on the semiconductor switch element 101 and the second pulse which turns on the semiconductor switch element 102, and returns the output voltage value of the DC-DC conversion device to the target value.
Further, when at a high input voltage and under a light or moderate load, the ON duty is small, as shown in FIG. 4B. Herein, when the ON duty is small and the period in which the semiconductor switch element 101 is turned on is short, a charging voltage of the resonating capacitor 4 when the period finishes decreases. Because of this, when the semiconductor switch element 102 is turned on, the voltage V21 applied to the primary winding 21 of the transformer 2 decreases, and it is not possible to generate a voltage enough to turn on the diodes 133 and 132 and cause a charging current to flow to the smoothing capacitor 5. Because of this, the currents I132 and I133 of the diodes 132 and 133 is 0 in the period in which the semiconductor switch element 102 is turned on, as illustrated in FIG. 4B. Consequently, the current I102 flowing through the primary winding 21 of the transformer 2 via the semiconductor switch element 102 is kept low. Further, when the semiconductor switch element 101 is turned on, a differential voltage between the input direct current voltage and the voltage V4 of the resonating capacitor 4 is applied to the primary winding 21 of the transformer 2 and the resonating reactor 3. At this time, as the voltage V21 of the primary winding 21 of the transformer 2 is a sufficient size of voltage, a voltage enough to turn on the diodes 131 and 134 and cause a charging current to flow through the smoothing capacitor 5 is generated in the secondary winding 22 of the transformer 2. Because of this, the large currents I131 and I134 flow via the diodes 131 and 134 in the period in which the semiconductor switch element 101 is turned on, as illustrated FIG. 4B. Consequently, when the semiconductor switch element 101 is turned on, a current flowing through the primary winding 21 of the transformer 2 via the semiconductor switch element 101 increases greatly in a linear fashion. Consequently, a breaking current flowing when the semiconductor switch element 101 is turned off does not decrease so much despite when under a light or moderate load.
A description has heretofore been given with the case of a high input voltage and a light or moderate load as an example, but the same problem also arises in the case of each of a high input voltage and a light or moderate load.
As above, the heretofore known DC-DC conversion device has the problem that the breaking currents of the semiconductor switches of a circuit on the primary side of the transformer increase, and power conversion efficiency decreases.